1. Field of the Invention
This invention relates to digital subscriber lines (DSL) and to smart systems for implementing Long reach Digital Subscriber Lines (LDSL).
2. Description of Related Art
High level procedures for meeting stated objectives for Long reach Digital Subscriber Line (LDSL) transmissions are disclosed. Some objectives for LDSL have been defined in publications available from standards organizations such as the International Telecommunications Union (ITU). For example, ITU publications OC-041R1, OC-045, OC-073R1, OJ-030, OJ-036, OJ-060, OJ-061, OJ-062, OJ-200R1, OJ-200R2, OJ-201, OJ-60R1, OJ-60R2 and OJ-210 set forth some LDSL objectives. Other objectives, standards and criteria for LDSL are also possible and may be accommodated by the disclosed inventions.
One LDSL target objective is to achieve a minimum payload transmission of 192 kb/s downstream and 96 kb/s upstream on loops having an equivalent working length of 18 kft 26 gauge cable in a variety of loop and noise conditions. One difficulty in achieving these target transmission rates is the occurrence of crosstalk noise.
The crosstalk noise environments that may occur for the above bit rate target objective are varied. For example, noise environments may include Near-end cross talk (NEXT), Far-end cross talk (FEXT), disturbance from Integrated Services Digital Networks (ISDN), High Speed Digital Subscriber Line (HDSL), Symmetric High-Bitrate Digital Subscriber Line (SHDSL), T1, and Self-disturbers at both the Central Office (CO) and Customer Premise Equipment (CPE) ends. NEXT from HDSL and SHDSL tend to limit the performance in the upstream channel, while NEXT from repeated T1 Alternate Mark Inversion (AMI) systems tend to severely limit the downstream channel performance. An additional source of noise is loops containing bridged taps that degrade performance on an Asymmetric Digital Subscriber Line (ADSL) downstream channel more so than the upstream channel.
Another drawback of existing systems is that it appears very difficult to determine a single pair of Upstream and Downstream masks that will maximize the performance against any noise-loop field scenario, while ensuring spectral compatibility and, at the same time, keeping a desirable balance between Upstream and Downstream rates.
One approach for LDSL relies on different Upstream and Downstream masks exhibiting complementary features. Realistically, all these chosen masks are available on any LDSL Platform. At the modem start up, based on a certain protocol, the best Upstream-Downstream pair of masks is picked up. Whether the best pair is manually chosen at the discretion of the operator, or automatically selected, this concept is identified as “smart DSL for LDSL”.
There are many reasons to implement smart DSL. For example, non-smart DSL systems may implement a single mask for upstream and downstream transmissions. A drawback with this approach is that the use of a single mask may prevent LDSL service in areas of the United States dominated by T1 noise.
In addition, the use of a single mask is a drawback because the existence of other spectrally compatible masks cannot be ruled out. LDSL service providers will want to have access to an array of mask/tools provided they are spectrally compatible. Service providers may decide to use only one mask according to the physical layer conditions, or any combination of masks for the same or other reasons.
Another advantage of Smart DSL is that it is a good way to handle providing LDSL services in different countries. For example, so far, LDSL work has focused on requirements set forth by SBC Communications (hereinafter “SBC”). As a result, it is risky of, for example, a US-based LDSL provider to rely on the ability to apply any masks that pass SBC tests to Europe, China or Korea. LDSL is a difficult project and essential for all the countries. Therefore, any scheme for LDSL standardization that takes into account merely SBC physical layer and cross talk requirements may jeopardize the ADSL reach extension in non-standard LDSL countries. Other drawbacks of current systems also exist.